Working Memory Capacity and Aphasia

The talk presents a capacity theory of syntactic comprehension disorders in aphasia. The work described was done in collaboration with Patricia Carpenter and Akira Miyake.

The theory assumes that aphasic patents still possess the structural (syntactic) and procedural knowledge necessary to perform syntactic analysis, but that they suffer from reductions in working memory capacity for language. The theory explains how reductions in working memory capacity can lead to the pattern of comprehension breakdown observed in aphasic patients. According to a resource-reduction view of comprehension impairments in aphasia, patients are assumed to have intact structural and procedural knowledge to parse various sentences, but suffer from consequences of severely reduced working memory resources. Two types of experiments provide relevant supporting data.

One way that the empirical predictions of the theory were tested was in several sentence comprehension experiments involving self-paced reading in aphasic patients. For example, the experiments examine how the reading performance (accuracy and distribution of word-by-word reading time) is affected by provision or withdrawal of contextual support, or increase in sentence complexity. Aphasic patients slow down their reading precisely on those parts of a sentence that are computationally most demanding, such as the verbs of transitive reversible sentences. Moreover, if the reading task is made more demanding, for example by withdrawing some contextual support, then the reading selectively slows down at the most demanding points in the sentence. So the on-line reading processes reflect the momentary demands on resources, and in particular, indicate that these patients difficulties occur when the demand on resources is high relative to the supply.

Another way to test this hypothesis is to put normal adults under resource-demanding situations and observe whether they show a pattern of comprehension errors similar to aphasic patients (Miyake, Carpenter, & Just, in press). College students read sentences (the same types used to test aphasic comprehension), using rapid-serial-visual-presentation (RSVP). The effect of syntactic complexity on comprehension accuracy in normals strongly resembles that in aphasics under RSVP conditions. …

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